It was all because one agent in one office in Phoenix wrote a memo.
This is NOT the FBI declaring QAnon a terrorist group.
It's just some dude whipping out a TPS report.
https://www.yahoo.com/entertainment/fbi-declared-qanon-domestic-terrorism-181448533.html
Distributed on May 30th, the memo from the bureau’s Phoenix office notes that it is the first report of its kind to take aim at “conspiracy-driven domestic terrorism,” citing a number of violent incidents the bureau believes were linked to conspiracy theories. “The FBI assesses these conspiracy theories very likely will emerge, spread, and evolve in the modern information marketplace, occasionally driving both groups and individual extremists to carry out criminal or violent acts,” the document says.